Wide band stub line device



y 1@950 ,H. K. CLARK 2,516,505

WIDE BAND STUB LINE DEVICE Filed Feb. 18, 19 4e EJECTOR UNIT INVENTOR. HUGH K. CLARK A T TOR/V5 Y Patented July 25, 1950 WIDE BAND STUB-LINE DEVICE I "Hugh K. Clark, Wayneshoro, Va., assignor to the United States of America: as represented by the Secretary of War Application February 18, 194aseria1m. 648,545

This' invention relates generally to electrical circuits and more particularly to a means iorinjecting voltages of other than radio frequencies into a coaxial radio-frequency transmission line.

It is frequently desirable to utilize an RL-F, transmission line to carry,.in addition to its normal radio-frequency signal, an additional di-,

Claims. Cl. 17844) rect current or a power-frequency current, for

example, one of 60 cycles-per second. These cur-.- rents do not interfere in any way with the normal radio frequency current of the line; however, if certain precautions are not taken at thepoints of injection and removal of the current from the line, the standing wave ratio of the'linemay be greatly increased due to a mismatch of impedance at these points. I

It is therefore an object of the present invention to provi'de'a means for injecting into and removingfrom a coaxial 'radio-frequency transmission line a direct or power-frequency current, It is a further object to render this means phys-f ically small. It is still another object so to design this means that the standing wave ,ratio of the line remains low over aband of frequenciesf,'

and so that radio-frequency power losses from the line over this frequency bandare not appre l,

v Other objects, features and advantages off this invention will suggestthe'mselves vto thoseskilled in'the art and will become apparentfrom the following description of'the invention taken in connection with the accompanying drawing'which shows a circuit diagram for a deviceof the type contemplated the invention.

' In the drawing there is shown a radio-frequency coaxial transmission line consisting of an inner conductor I0 and an outer conductor l2. Points l4' and IS on conductor I!) are the input and out-- put points, respectively, for the direct or lowfrequency voltage, while capacitors it and 20 are radio-frequency bypass capacitors inserted in the centralconductor [0 to permit the flow of radio frequency current while preventing the flow "of low-frequency or direct current through R.-'F. terminals 22 and 24. The injector unit itself in cludes a first section of coaxial line consisting of' anouter conductor 26 and a coiled inner iconductor 28, the latter being connected to conductor the first and second sections being 3:1 or less.- At the junction of the two sections, there is placed a loading consisting of four equal resistors 34, each in series with a capacitor, connected between the inner and outer. conductors of,the line sections. Capacitors 36 are R.-F. bypassjcae pacitors; that is, they offer low ,impedance'jo radio frequencies but high impedance to D.-C. or low frequencies. The values of the resistors. are made suchthat their parallel resistance is approximately equal to the magnitude of the characteristic impedance" of the line sections; which is almost entirely resistive. An R.-F. by-Q pass or shorting capacitor 38 isconnected between the inner and outer conductors of the second'line section at a point near the end thereof. The 11-0, or low-frequency input is applied to terminals 40, one connected to the central conductor of the line section and the other being grounded and connected to the outer conductorthereof. An ejector unit 42 identical with the injecto'rlunit just described is connected between point"; on the inner conductor of the transmission line and the output terminals through which the direct or low-frequency voltage'isremoved. Since capacitor 38 presents a negligible impedance to'radio frequency, the injector line section without the loading resistors would appear "elec trically as ashort-circuited stub in parallel with themain transmission line. At the frequency at which its total electrical length is one-fourth wavelength, the stub would present a very high impedance and have little effect on the main transmission line; however, at other frequencies its impedance is less, and it serves to increase the standing wave ratio von the line.

stubis one half wavelength, itwo'uld offer a very istic impedance of ohms, while that 'of the stub is 189 ohms, the standing wave ratio on the line lessjthan or equal to 2:1 over a. frequency band whose upper and lower limits are in iail a tl o In order to increase theuseable frequency band.

and to avoid the short circuiting' effect caused when the stub is one-half wavelength, th stub is loaded withits characteristic impedance'ata' pointwhich is at least one-fourth of the distance from the shorted end of the stub, to thepointof connection to, the main line. This resistance has relatively little effect at lower frequencies, being in parallel with an inductance whose impedance is much smaller, butat the" frequency at which v At the rte-j quency at which the total electrical length of the the total length of the stub is one-half wavelength, because of this loading, the impedance presented by the stub will have an appreciable value.

It has been determined experimentally that if the loading is too close to the shorted end of the stub, at the frequencies at which the stub length is one-half, ..wavelength the stub presents an extremely l'ow impedance, Moving the load away from the shorted end increases the impedance presented. This impedance, as stated above, becomes appreciable when the loading occurs at a point which is at least one-quarter of the total stub length away from the shorted, end. However, increasing this loading distance from the shorted end also has the efiect of lowering the highest frequency at which the stub can be used, which occurs when the distance between the loading point and the short circuited end is one-half wavelength.

As an example, assuming the resistiv load is one fourth oi the distancefrom the end, of the stub and that the values of characteristic resistance are as given above, when the stub is one.- half wavelength long the impedance of thestub would be 72-i36"ohms at the injection point. Underj the se' conditions a standing wave, ratio of ;.1.(,i is'produeed on the transmission line, and..-about 60 per cent of the radioefrequency power; sent down the line is dissipated in theload'; The remainder is either reflected or dissipated 'inlthe' stub load resistance. Since the stub with resistorload is effective not only, at the frequency at which it is one-half wavelength long but also atwhigher and lower frequencies, the bandwidth,oflrtl'iedevice is improved considerably by, resistivelloading. ,The upper limitof the frequency-band is inthis case determined by the frequenc jat'yvhichthe distance from th load resistors .to theshorting capacitor is one-half wavel ngth; Thepurpose. of the coiled inner condoctor is to decrease-the size oi the stub injector unit to about..one-siXth the length of a unit in which the inner. conductor is straight. The purpose, of, usingfour load resistors. .is to. minimize the .efiective inductance of the resistive. load. Capacitors 36act-as short circuits for radio frequency while/preventing the direct or low-frequency voltage from being short-circuited to ground by the rather low, value resistors 34. Ob-

viously unit 42,;whichis identical-with the unit whoseoperation has just been described, may be used to remove the direct or power-frequency: current from the transmission line.

, While there has been described what isatpres;

entLcWiSidredto be the preferred embodimentof this 'inyentionit will be, obvious to-tthose skilled in the art that'varipus changesand modifications may bemadetherein without departing from the sccpejof,therinvention. M

The invention claimed is:

transmission line having ,an outer conductor, an inner conductor-and input and outputterminals; first and'second coaxial line sections connected substantially in parallel with said transmission line at, two given point thereof, each of said line sections-having a coiled inner conductor; two

radio-irequency-bypass capacitors, one-f said;

capacitors connected between the inner and outer conductors, of each, of said line sections at the terminationsthereof; a plurality of resistors connected totheinner conductorof each of said line sections, at'a point at least one-quarter of the distance ifrom the radio frequency shorted end there.

1. In ,combinationya, coaxial radio-frequency of to the point of connection thereof to said transmission line, the distance of said point from said radio frequency shorted end also being less than an integral multiple of one half Wavelength of the radio frequencies being transmitted by said transmission line, the parallel resistance of said resistors being approximately equal to the characteristic resistance of the line section in which they are connected; a plurality of radio-frequency bypass capacitors each one connected respectively between each one of said plurality of resistors and the outer conductors of said line sections; two radio-frequency bypass capacitors inserted in the inner conductor of said transmission line between said input and output terminals respectively and the remainder of said inner condoctor; and a plurality of means adapted to apply direct and low-frequency voltages between the inner and outer conductors of said first line section at the termination thereof, a plurality of means adapted to remove direct and low frequency output voltages from between the inner and outer conductors of said second line section at, thetermination thereof.

2.,,In combination, a coaxial radio-frequency transmission line having an outer conductor, an inner conductor and input and output terminals,

, a plurality of pairs of coaxial line sections connecte'd substantially in parallel with said transmission line, each of said pairs including a first sectimi and asecond section, two radio-frequency bypass apacitors inserted in the inner conductor of said transmission, line between said input and output terminals respectively and the remainder of saidinner conductor, a pluralityof radio-frequency bypass capacitors connected respectively between the inner and outer conductors of each of said line, sections at the terminations thereof, resistive load means electrically connected to each of said linesections at a point at least oneqllarter of the distance from the termination thereof to theconnection thereof to said transmissionline, the distance of said point from the said termination. also being less than an integral multiple of. one half wavelength of the radio frequencies being, transmitted, and a plurality of means adapted to appl direct and low-frequency voltagesbetween theinner and outer conductors of saidtfirst .line sections at the terminations thereof ,a plurality ofmeans to remove direct and low frequency output voltages between the inner and, outer conductors of said second line sections.

3. A wide band stub line device for a radio frequency coaxial transmission line, comprising lcoaxial conductor having an outer conductor d Coiled. inner conductor, a. first end of said coaxial conductor being adapted to electrically couple to said transmission line, a radiofrequency bypass capacitor connectedat a second endof said coaxial conductorbetween its outer and inner conductors, means at said second end to receive direct and low frequency voltages, resistive load means having a resistance substantially equal to thecoaxial-eonductor characteristic resistance, Said means-being electrically connected between saidcoiled inner conductor and said outer conductor at a point at least one quarter of the total length of the coaxial conductor from its second end, the distance from said point to the second endof said coaxial conductor being less than an. integralamultiplecf one half wave length of the radio frequencies with which the devic i to be used. I

4. A Wide band stub line device as defined in 1 41- 3. w e ein sai resistive load me ns coinprises at least one resistor connected at one end REFERENCES CITED to the said point on the coiled inner conductor and at least one radio frequency bypass capacitor connected between the other end of said resistor and the outer conductor.

5. A wide band stub line device as defined in The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date claim 3, wherem said second end having the radlo frequency bypass capacitor is terminated in 2006994 Hopkms July 21 1935 means to remove direct and low frequency voltages. 10

HUGH K. CLARK. 

